The term "liquid synthetic fuels" has come to refer to prepared liquids having characteristics approaching those of the petroleum-based fuels in current commerce, specifically gasolines, kerosenes, jet fuels, and fuel oils. Synthetic fuels may be prepared by processes comprising liquefaction of coals, destructive distillation of kerosene or coal, and extraction or hydrogenation of organic matter in coke liquids, coal tars, tar sands, or bitumen deposits, in addition to processes that fit the classical definitions of synthetic processes.
Carbonaceous fuel substances, other than natural petroleum, are widely distributed throughout the world. In the Western hemisphere, Canada has large tar sand, bitumen (or very heavy crude oil), and coal deposits. The United States has very large reserves of coal and shale. The total resource base is immense by any estimate. Unfortunately, estimates of the quantities of in-place reserves that may be recoverable with current technology range no higher than about 10%. Hence, considerable pressure for new recovery technology will develop along with pressure for advances that reduce the level of fuel usage.
Most carbonaceous substances may be combusted more or less directly in suitable equipment to produce heat or power. Direct combustion may represent the means for deriving maximum benefit of the heating or energy value of particular fuel substances, although the necessity to limit emissions of noxious compounds, especially of sulfur or nitrogen, and to control disposition of noncombustible residues, may significantly reduce the overall thermal efficiency of the combustion process and add to the cost of work so produced. The principal concern regarding the air pollutants emitted from the combustion of fossil fuels, for instance, is the long-term chronic health effects that appear to be related to large concentrations of air pollutants. In addition, the sulfur or nitrogen oxides (or both) are believed to be the cause of acid rain.
Due to these concerns, the Environmental Protection Agency is currently enforcing very strict emission regulations for truck and diesel engines. Similarly, strict emissions standards will go into effect for agricultural and other off-road vehicle engines starting in 1996. These standards are applied with particular scrutiny to particulate emissions. Particulate emissions are especially troublesome in that they represent a more significant and present threat to the environment; one which is readily observable through effects such as smog. Researchers are attempting to maintain fuel efficiency while reducing the percentage of particulate emissions. Recently, advances have been made which strike a more desirable balance through the use of blends of petroleum based fuel with alkyl esters of the fatty acids contained in vegetable oils or animal fats, known as "biodiesel". Alkyl esters, such as methyl esters, of fatty acids are generally preferred in biodiesel over the use of the vegetable oils and animal fats themselves because the alkyl esters have a viscosity that is more appropriate to diesel fuel. Through the use of these fuel blends, researchers have attained reductions in particulate emissions from diesel engines.
Although advantageous in reducing particulate emissions, the use of biodiesel is not without drawbacks. In particular, saturated long-chain fatty acid esters that result from the esterification of the fatty acids contained in animal fats and vegetable oils with methanol or ethanol tend to raise the freezing points of the fuel blends, which causes the fuels to precipitate or gel at cold temperatures. Keeping such blends liquid and suitable for engine use requires special equipment and handling. These problems greatly reduce the practicality of using fatty ester-petroleum distillate blends in cold weather.
For example, methyl or ethyl esters of soybean oil fatty acids have cloud points (the lowest temperature at which a fluid can be held without beginning to crystallize and turn cloudy) of about 1.degree.-5.degree. C. and pour points (the lowest temperature at which a fluid can be held without crystallizing to the extent that it lacks sufficient fluidity to be poured) of about -4.degree. C. Furthermore, when mixed with diesel fuel, slightly lower temperatures can be tolerated (i.e., -12.degree. C. when using methyl esters of soybean oil fatty acids, as compared with -21.degree. C. for winter grade diesel fuel).
In U.S. Pat. No. 4,364,743 (Erner et al.), various synthetic vegetable oil esters are disclosed as possible fuel additives. This patent discloses that the combustion properties of the blends are satisfactory. However, it does not disclose that such esters can alleviate the tendency of the fuel blends to precipitate or gel at cold temperatures, while retaining suitable combustion properties.
Therefore, there is a need for a method to produce vegetable oil ester-petroleum distillate fuel blends that exhibit reduced crystallization temperatures, thereby enabling wintertime usage.